In high speed on-the-fly line printers a plurality of print hammers usually arranged in a row are selectively operated to strike the type faces on a constantly moving type carrier. The type carrier may be a revolving flexible band, belt, chain or train or a rotating drum. The print hammers are generally operated electromechanically preferably using electromagnetic actuators including an armature which when the electromagnet is energized, i.e. fired, propels an impact element or hammer from a rest position to the point of impact. Commonly, the armature stroke is stopped, i.e. seals, before impact while the hammer element continues in free flight to the point of impact. At the instant of impact, the hammer rebounds to be restored to the rest position where after a brief period of settling comes to rest ready to be fired.
Good registration of the printed characters requires that hammers be controlled so that impact occurs at the exact time that the desired characters become aligned with the selected print hammer/print position. It is further desirable to be able to terminate the energization of the electromagnet at or slightly after the time the armature seals, thereby saving energy and to be able to accommodate for the period during which hammers are settling in preparation for repeat firing. It is also desirable that the flight times be easily changed to accommodate variances in hammer operating characteristics during a relatively extended use period and that these changes be made without the need for altering control circuitry.
Various control schemes have been devised for operating print hammers to compensate or adjust for variation in the actual flight times of the print hammers due to variances in printer operating characteristics. Basically these control schemes introduce variable delay circuits into the hammer fire circuitry. While some of these systems may largely dispense with the arduous and time consuming task of manually adjusting hammer flight time, they are essentially inadequate for achieving reliable precision hammer flight control required for very high printing speeds, e.g. where the type carrier speeds greatly exceed 300 inches per second. Also they lack the capability to be easily and readily adapted to control the time for terminating the energization of the electromagnet and/or to make accommodation for the settling time of the hammers before they are again fired. Most prior art control schemes require complex timing controls and/or require changes in circuitry or circuit components to make the adjustments which compensate for changes in the operating characteristics.